codex-tools: extract code mode tool spec adapters (#16132)

## Why

The longer-term `codex-tools` migration is to move pure tool-definition
and tool-spec plumbing out of `codex-core` while leaving session- and
runtime-coupled orchestration behind.

The remaining code-mode adapter layer in
`core/src/tools/code_mode_description.rs` was a good next extraction
seam because it only transformed `ToolSpec` values for code mode and
already delegated the low-level description rendering to
`codex-code-mode`.

## What Changed

- added `codex-rs/tools/src/code_mode.rs` with
`augment_tool_spec_for_code_mode()` and
`tool_spec_to_code_mode_tool_definition()`
- added focused unit coverage in `codex-rs/tools/src/code_mode_tests.rs`
- rewired `core/src/tools/spec.rs` and `core/src/tools/code_mode/mod.rs`
to use the extracted adapters from `codex-tools`
- removed the old `core/src/tools/code_mode_description.rs` shim and its
test file from `codex-core`
- added the `codex-code-mode` dependency to `codex-tools`, updated
`Cargo.lock`, and refreshed the `codex-tools` README to reflect the
expanded boundary

## Test Plan

- `cargo test -p codex-tools`
- `CARGO_TARGET_DIR=/tmp/codex-core-code-mode-adapters cargo test -p
codex-core --lib tools::spec::`
- `CARGO_TARGET_DIR=/tmp/codex-core-code-mode-adapters cargo test -p
codex-core --lib tools::code_mode::`
- `just bazel-lock-update`
- `just bazel-lock-check`
- `just argument-comment-lint`

## References

- #15923
- #15928
- #15944
- #15953
- #16031
- #16047
- #16129

codex-rs/tools/README.md

@@ -21,6 +21,7 @@ schema and Responses API tool primitives that no longer need to live in
 - `ResponsesApiWebSearchUserLocation`
 - `ResponsesApiNamespace`
 - `ResponsesApiNamespaceTool`
+- code-mode `ToolSpec` adapters
 - `parse_tool_input_schema()`
 - `parse_dynamic_tool()`
 - `parse_mcp_tool()`
@@ -30,6 +31,8 @@ schema and Responses API tool primitives that no longer need to live in
 - `dynamic_tool_to_responses_api_tool()`
 - `mcp_tool_to_responses_api_tool()`
 - `mcp_tool_to_deferred_responses_api_tool()`
+- `augment_tool_spec_for_code_mode()`
+- `tool_spec_to_code_mode_tool_definition()`
 
 That extraction is the first step in a longer migration. The goal is not to
 move all of `core/src/tools` into this crate in one shot. Instead, the plan is